The present invention relates to a shutter and lowlight indicator actuating device for a camera.
In electronic cameras, it is desirable to provide a low-light indicator lamp which is energized when the incident light is below a certain value, the lens aperture is fully open and the computed shutter opening time is long enough that there is danger of blurring the picture by moving the camera when the camera is hand-held. It is further desirable to maintain the lamp on until the exposure is completed, especially with exposures of over one second. Various devices comprising switches have been devised to perform this function. However, switches are bulky and are not compatible with the miniaturization of electronic cameras. Furthermore, switches are subject to mechanical failures such as caused by increased contact resistance resulting from prolonged use.
A device has been proposed for use in the KODAK INSTAMATIC 60 (trade name) camera in which an input for a Schmitt trigger for controlling a low-light indicator lamp is connected to a battery terminal through another Schmitt trigger which is used for actuating the shutter so that the lamp is lighted while the shutter is open. Whereas this device is generally effective, it suffers from a serious drawback in that the internal resistance of the Schmitt trigger for the shutter varies in dependence on ambient conditions such as temperature, humidity and the like to such a degree that the operation of the Schmitt trigger for the indicator bulb is erratic, especially under low-light conditions when the operation must be exact.
More specifically, the Schmitt trigger for the shutter comprises an output transistor, the input of the Schmitt trigger for the low-light indicator being connected to the collector of this output transistor. The Schmitt trigger for the low indicator is enabled when the Schmitt trigger for the shutter produces a low output, or when the output transistor is in saturation. This low output is used as a ground reference for the low-light sensing function.
However, a small variation in the resistance of the output transistor when operated in saturation, which is within the normal manufacturing tolerances of such transistors even when fabricated as integrated circuits, will cause a considerable variation in the light level at which the low-light indicator is activated. Other conditions which can considerably change this level are the voltage drop across a battery used to power the device which normally occurs with use and variations in temperature and humidity.
An attempt to overcome this effect has been proposed which involves using the output transistor collector voltage of the Schmitt trigger for the shutter as a ground reference for both the low-light sensing function and also for the power supply of the Schmitt trigger for the low-light indicator. Whereas this is a partial solution to the problem due to the compensating effect of having any battery voltage variations applied equally to the sensing and power supply functions of the Schmitt trigger for the low-light indicator, in a practical circuit the low-light indicator lamp, which is most advantageously provided in the form of a light-emitting diode (LED), is connected in series with its current limiting resistor in parallel with the light sensing circuitry of the Schmitt trigger for the low-light indicator.
The LED tends to act as a constant voltage device, with the result that the voltage drops across the output transistor and the current limiting resistor vary non-linearly as the battery voltage decreases. This produces a non-linear variation of the low-light indicator activation level. The effect is reduced by making the value of the current limiting resistor very high compared to the internal resistance of the LED. However, yet another undesirable effect is introduced thereby in that the LED current and brightness are severely reduced and the rate of decrease in brightness of the LED is greater than the rate of decrease of the battery voltage. For these reasons, the expedients described above do not overcome the problem and said problem has remained heretofore unsolved.